Nearly 1 billion people may be vitamin D-deficient due to disease or overuse of sunscreen, according to a report published in the Journal of the American Osteopathic Association.

Vitamin D is a fat-soluble hormone that plays a vital role in many bodily functions. Insufficient levels can lead to heart disease, diabetes, high blood pressure, arthritis and more. It is produced naturally by the body during sun exposure.

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"People are spending less time outside, and when they do go out, they're typically wearing sunscreen, which essentially nullifies the body's ability to produce vitamin D," said Dr. Kim Pfotenhauer, assistant professor at Vallejo, California-based Touro University. "While we want people to protect themselves against skin cancer, there are healthy, moderate levels of unprotected sun exposure that can be very helpful in boosting vitamin D."

Healthy vitamin D levels can be obtained by spending five to 30 minutes outside at least twice a week between the hours of 10 a.m. to 3 p.m. But skip the sunscreen - products with SPF 15 or higher decrease vitamin D3 production by 99 percent.

"You don't need to go sunbathing at the beach to get the benefits," explained Dr. Pfotenhauer. "A simple walk with arms and legs exposed is enough for most people."

Vitamin D can be found in foods like salmon, catfish, shiitake mushrooms, eggs and tofu. However, chronic conditions like type 2 diabetes, kidney disease, celiac disease and Crohn's disrupt the body's ability to absorb vitamin D from food.

Medical experts hope to determine if lack of the hormone contributes to autoimmune disorders, respiratory disease, cancer and multiple sclerosis.

"Science has been trying to find a one-to-one correspondence between vitamin D levels and specific diseases," added Dr. Pfotenhauer. "Given vitamin D's ubiquitous role in the body, I believe sufficient vitamin D is more about overall health."